FIGS. 18 and 19 are cross-sectional views of a conventional disk cartridge 51. The disk cartridge 51 is composed of an upper shell 52 and a lower shell 53, which are molded components (synthetic resin molded components; hereinafter referred to as molded components). A disc 54, which is a molded component and a disk recording medium, such as a 12 cm optical disk including a DVD or a DVD-ROM, or a magneto-optical disk, is horizontally accommodated inside the disk cartridge 51 so as to be rotatable and movable vertically (in the directions of the arrows a and b).
An oblong opening 53a is formed in the lower shell 53 from a substantial center to a front edge thereof, and a turntable 61 and an optical pickup (not shown) described later are inserted from the bottom of the opening 53a. A chucking plate 55 composed of a ferromagnetic material such as stainless steel and formed by presswork is arranged on or above the middle of the disk 54 inside the disk cartridge 51. The chucking plate 55 is held by a plate supporter 56 of a molded component or the like below the upper shell 52 so as to be rotatable and movable vertically (in the directions of the arrows a and b).
In this case, the chucking plate 55 has a disk shape. A central circular recess 55a projecting upward is formed concentrically in the center of the chucking plate 55. A disk pressing portion 55b projecting downward is formed concentrically outside the central circular recess 55a. A tapered portion 55c having a diameter gradually increasing with height is formed concentrically outside the disk pressing portion 55b. A horizontal peripheral flange 55d is formed around the tapered portion 55c. 
The plate supporter 56 is an integrated component including a peripheral flange 56a, a cylindrical rib 56b projecting downward from an inner edge of the peripheral flange 56a in the vertical direction, and an inner flange 56c formed concentrically and horizontally at an inner region defined by the bottom end of the cylindrical rib 56b. 
The inside diameter of the cylindrical rib 56b of the plate supporter 56 is larger than the outside diameter of the chucking plate 55. The peripheral flange 55d of the chucking plate 55 is in advance disposed in an inner region defined by the cylindrical rib 56b, and the peripheral flange 56a of the plate supporter 56 is horizontally mounted on a lower (inner) surface 52a of the upper shell 52 by ultrasonic welding or the like. The chucking plate 55 is therefore held so as to be rotatable between the upper shell 52 and the inner flange 56c of the plate supporter 56 and movable vertically (in the directions of the arrows a and b).
As shown in FIG. 18, when the disk cartridge 51 is not used, the disk 54 moves down in the direction of the arrow a under its own weight. As a result, the disk 54 is horizontally placed on the lower shell 53 such that the lower surface 54a at an outer region around a center hole 54b of the disk 54 comes into contact with the upper (inner) surface 53b of the lower shell 53 at an inner rib 53c molded integral with an outer region of the opening 53a and a data recording region 54c is disposed away from the upper surface 53b of the lower shell 53.
The chucking plate 55 also moves down in the direction of the arrow a under its own weight so that the tapered portion 55c of the chucking plate 55 comes into contact with an inner edge of the inner flange 56c of the plate supporter 56 from above and thereby centering the chucking plate 55. The peripheral flange 55d of the chucking plate 55 is thus horizontally placed on the inner flange 56c of the plate supporter 56. A gap G11 is formed between a lower surface of the annular disk pressing portion 55b, which is the lowest part in the chucking plate 55, and an upper surface 54d of the disk 54.
As shown FIG. 19, when the disk cartridge 51 is loaded into a disk drive device (not shown), the turntable 61 driven by a spindle motor (not shown) and the optical pickup (not shown) are inserted into the opening 53a of the lower shell 53 from below relatively in the direction of the arrow b. A centering projection 61a, which is formed in the central upper part of the turntable 61 and has the shape of a substantially truncated cone, is inserted into the center hole 54b of the disk 54 from the direction of the arrow b. The disk 54 is thus centered and the lower surface 54a of the disk 54 at the outer region around the center hole 54b is horizontally placed on an annular disk mounting face 61b that is horizontally formed in an outer region of the turntable 61. The disk 54 is then moved up relatively in the direction of the arrow b to a substantial middle position between the upper shell 52 and the lower shell 53 in the disk cartridge 51 by the turntable 61. As a result, the disk 54 is supported not in contact with the upper shell 52 or the lower shell 53, and the optical pickup approaches the lower surface 54a of the disk 54.
At this time, the disk 54 is moved up in the direction of the arrow b by the turntable 61 so that the upper surface 54d at the outer region around the center hole 54b of the disk 54 comes into contact with the disk pressing portion 55b of the chucking plate 55 from below and thereby moving this chucking plate 55 up relatively in the direction of the arrow b. Additionally, at this time, a circular chucking magnet 62, which is mounted into the center of the centering projection 61a of the turntable 61, is inserted into an inner region defined by the central circular recess 55a of the chucking plate 55 from the direction of the arrow b, and the chucking magnet 62 approaches the chucking plate 55 from below.
The chucking plate 55 is then attracted in the direction of the arrow a by magnetic attraction of the chucking magnet 62. As a result, the annular disk pressing portion 55b of the chucking plate 55 crimps the upper surface 54d of the disk 54 at the outer region around the center hole 54b onto the annular disk mounting face 61b of the turntable 61 from the direction of the arrow a and fixes it, i.e., the disk 54 is chucked.
The chucking of the disk 54 allows the tapered portion 55c and the peripheral flange 55d of the chucking plate 55 to be not in contact with the cylindrical rib 56b or the inner flange 56c of the plate supporter 56. The disk 54 is then driven to be rotated by the turntable 61 driven by the spindle motor inside the disk cartridge 51. During rotation of the chucked disk in which data is recorded to and/or played back from the data recording region 54c of the disk 54 by the optical pickup, the chucking plate 55 is rotated together with the disk 54 in such a way that the chucking plate 55 is not in contact with the upper shell 52 or the plate supporter 56.
During this rotation of the chucked disk, four gaps are defined by the disk 54, the chucking plate 55, the upper shell 52, and the plate supporter 56: a gap G12 is defined between the edge of the peripheral flange 55d of the chucking plate 55 and an inner surface of the cylindrical rib 56b of the plate supporter 56 in the horizontal direction; a gap G13 is defined between the peripheral flange 55d and the lower surface 52a of the upper shell 52 in the vertical direction; a gap G14 is defined between the peripheral flange 55d and the inner flange 56c of the plate supporter 56 in the vertical direction; and a gap G15 is defined between the inner flange 56c and the disk 54 in the vertical direction.
However, in the conventional disk cartridge 51, the structure in which the upper shell 52 holds the chucking plate 55 by the plate supporter 56 mounted on the lower surface 52a of the upper shell 52 by ultrasonic welding or the like requires a molding step of the plate supporter 56 and a mounting step by ultrasonic welding, thus incurring additional cost.
Additionally, as shown in FIG. 19, since the inner flange 56c of the plate supporter 56 mounted on the lower surface 52a of the upper shell 52 is placed between the disk 54 and the peripheral flange 55d of the chucking plate 55, it is necessary to consider the thickness B of the inner flange 56c of the plate supporter 56 within the inner thickness A defined between the lower surface 52a of the upper shell 52 and the upper surface 53b of the lower shell 53 of the disk cartridge 51 in the vertical direction.
In other words, as shown in FIG. 19, since the inner thickness A includes the thickness B of the inner flange 56c, gaps defined by the upper shell 52, the disk 54, and the chucking plate 55 in the vertical direction to be particularly considered during the rotation of the chucked disk are as many as three gaps G13, G14, and G15.
In proportion to the number of the gaps to be considered, each of the dimensions of the gaps must be reduced. Therefore, the sizes of these three gaps G13, G14, and G15 are inevitably reduced.
If these three gaps G13, G14, and G15 are significantly small, the disk 54 and the chucking plate 55 tend to come into contact with the inner flange 56c of the plate supporter 56 when the disk 54 and the chucking plate 55 are driven to be rotated together with the turntable 61. This results in decreased reliability. Additionally, since the upper shell 52, the lower shell 53, and the plate supporter 56 being molded components and the like shrink or expand with environmental variations, such as temperature or humidity variations, and the thickness of the disk 54 varies in a manufacture process, the reliability is further decreased.
One way to solve these problems is that the thickness C, which is the entire thickness of the disk cartridge 51, is increased so as to increase the dimensions of the gaps G13, G14, and G15. However, this way is not available when the thickness C, which is the external thickness of the disk cartridge 51, is predetermined by a standard.
The present invention is accomplished to solve the above-described problems. An object of the present invention is to provide a disk cartridge that does not require a plate supporter for holding a chucking plate on an upper shell.